Take a look at the narrative for P21S Total Auto Wash. I use it for plastic wheel wells and metal undercarriage on my cars with no agitation even though it says light agitation.
Yea the fabric is a bit of a pain. On my truck I spray them with a APC, let it dwell, then rinse them with a strong jet of water. It has been working so far. I do not dress them with anything after.
I use APC undiluted of the well is dirty with mud etc. Getting in there is problematic on my Audi as there's not a lot of room between the well and tire. Using the Mother's brush may be helpful in that regard, however the brush tips don't look all that sturdy.